The basic structure of a laterally diffused metal-oxide-semiconductor field-effect transistor using RESURF (reduced surface field) principle consists of a low-doped P-type substrate and a low-doped N-type epitaxial layer. A P well is formed on the epitaxial layer and N+, P+ impurities are implanted into the P well, such that a transverse P-well/N-epi junction and a longitudinal P-sub/N-epi junction are formed. Due to a higher doping concentration at both ends of the transverse junction, the breakdown voltage of the transverse junction is lower than that of the longitudinal junction. The basic principle of RESURF is to enable the epitaxial layer to be completely depleted before the transverse junction reaches the critical avalanche breakdown field by using the interaction of the transverse junction and the longitudinal junction. By reasonably optimizing the device parameters, the breakdown of the device occurs in the longitudinal junction, thereby playing a role in reducing the surface field.
By adjusting the concentrations of N-type and P-type impurities of a drift region to meet RESURF requirements, the conventional RESURF structure achieves a required withstand voltage. In addition, on-resistance is required to be as small as possible to reduce switching losses. A laterally diffused metal-oxide-semiconductor field-effect transistor (LDMOSFET) having the conventional RESURF structure is provided with a floating P-type ring in the drift region. In a switching process of the device, since the floating P-type ring cannot release the minority carrier charges generated in the previous switching cycle in time, the charge accumulation effect in the next cycle easily leads to a higher gate-drain capacitance (Cgd) and poor dynamic characteristics of the device.